1. Field of the Invention
The present invention generally relates to radio receivers and more particularly, to a radio receiver which improves transmission quality by compensating for channel distortion in a radio wave received via a plurality of branches and paths and combining compensated signals.
2. Description of the Related Art
Recently, the number of people subscribing to a radio communication system is increasing with the advancement in the liberatization of the market. Intensive studies are being undertaken to develop code division multiple access (CDMA) characterized by a high efficiency in using radio frequencies.
In a reception system in which CDMA is introduced, a high transmission quality is obtained by dividing and despreading a received wave arriving over a time longer than a time commensurate with a reciprocal of a signal band, compensating for a channel distortion (which occurs as a result of a variation in the amplitude and a phase rotation) in individual waves thus processed, and subjecting the waves to a predetermined combination process. A RAKE receiver is most suitably used in such a system.
FIG. 1 shows a construction of a RAKE receiver according to the related art.
Referring to FIG. 1, a power feed end of an antenna 61-1 is connected to inputs of despreaders 63-11, 63-12, 63-13 via a demodulator 62-1. Likewise, a power feed end of an antenna 61-2 is connected to inputs of despreaders 63-21, 63-22, 63-23 via a demodulator 62-2. Outputs of the despreaders 63-11, 63-12, 63-13 are connected to corresponding inputs of a combination processor 65 via channel distortion compensation units 64-11, 64-12, 64-13. Likewise, outputs of the despreaders 63-21, 63-22, 63-23 are connected to corresponding inputs of the combination processor 65 via channel distortion compensation units 64-21, 64-22, 64-23. A base band signal subject to signal detection is obtained at an output of the combination processor 65.
The channel distortion compensator unit 64-11 is constructed of a channel estimation unit 66-11, a delay unit 67-11 and a multiplier 68-11, outputs of the channel estimation unit 66-11 and the delay unit 67-11 being connected to an input of the multiplier 68-11 and an output of the multiplier 68-11 being connected to a corresponding input of the combination processor 65.
The channel distortion compensation units 64-11, 64-12, 64-13, 64-21, 64-22, 64-23 are constructed similarly. Therefore, suffixes 12, 13, 21, 22, 23 are used to designated those components in the channel distortion compensation units 64-12, 64-13, 64-21, 64-22, 64-23, respectively, that correspond to the channel estimation unit 66-11, the delay unit 67-11 and the multiplier 68-11. The description and illustration of those components are omitted.
According to the construction shown in FIG. 1, a radio wave arriving at the antennas 61-1, 61-2 is demodulated by the demodulators 62-1, 62-2, respectively, and subject to despreading process by a group of the despreaders 63-11, 63-12, 63-13 and a group of the despreaders 63-21, 63-22, 63-23, respectively, in accordance with the same spreading code. Thus, the received radio wave is separated into individual waves corresponding to respective paths formed in a radio transmission route.
A received radio wave will hereinafter be denoted by Y.sub.k,n, where n indicates a time series and k (which ranges between 1 and 6(=K)) indicates a number associated with one of the fingers formed by each of the despreaders 63-11, 63-12, 63-13, 63-21, 63-22, 63-23.
Each of the channel estimation units 66-11, 66-12, 66-13, 66-21, 66-22, 66-23 of the channel distortion compensation units 64-11, 64-12, 64-13, 64-21, 64-22, 64-23 respectively, retrieves a pilot signal contained in the divided received wave in a predetermined format so as to estimate a channel characteristic g.sub.k,n of the associated path by detecting an error of a signal point for the pilot signal.
Each of the multipliers 68-11, 68-12, 68-13, 68-21, 68-22, 68-23 carries out an arithmetic operation EQU Z.sub.k,n =g.sub.k,n *.multidot.Y.sub.k,n
where g.sub.k,n * is a complex conjugate of the channel characteristic.
The combination processor 65 obtains a base band signal by carrying out an arithmetic operation ##EQU1## on the results of the arithmetic operation by the multipliers 68-11, 68-12, 68-13, 68-21, 68-22, 68-23.
Since the channel distortion compensation is applied to individual paths, the base band signal Z.sub.n is given as a result of a maximum ratio combination of received signal dividends weighted in accordance with a S/N ratio. Such a base band signal has a maximum S/N and is fit for signal detection.
A description will now be given of a problem with the reception according to the related art.
Referring to FIG. 1, even when the spreading codes used by the despreaders 63-11, 63-12, 63-13, 63-21, 63-22, 63-23 in the despreading process are perfectly synchronized with the received wave, the channel characteristic g.sub.k,n of a lower power component in the received wave arriving on a path characterized by a significantly large propagation loss is estimated by the channel estimation units 66-11, 66-12, 66-13, 66-21, 66-22, 66-23 with a significantly low precision.
In a mobile communication system, the channel characteristic varies constantly as a mobile station moves between areas of different geographical conditions and different distributions of buildings, trees etc. It is also to be noted that a variety of multipaths are created in the mobile communication system. Therefore, the aforementioned low power component could occur relatively frequently.
It is thus likely that a noise is superimposed on the base band signal obtained at the output of the combination processor 65 due to an error in the estimation process. When the noise is superimposed, the precision of the subsequent signal detection is also lowered, thus causing the quality of communication to be degraded significantly.